1. Field of the Invention
The present invention relates to a method of preparing a bismuth oxide superconductor, and more particularly, it relates to a method of preparing a bismuth oxide superconductor, which can improve a magnetic field property of critical current density.
2. Description of the Background Art
In recent years, superconducting materials of ceramics, i.e., oxide superconducting materials, are watched as materials which exhibit higher critical temperatures.
For example, a bismuth oxide superconducting material, which exhibits a high critical temperature of about 110K, is expected as a practicable superconducting material.
It is known that a bismuth oxide superconductor has a phase showing a critical temperature of 110K and those showing critical temperatures of 80K and 10K. It is also known that a non-superconducting phase inevitably partially appears particularly when a 110K phase is to be formed.
In relation to such a bismuth oxide superconductor, it is further known that a 110K phase has a 2223 composition in a composition of Bi--Sr--Ca--Cu, or (Bi,Pb)--Sr--Ca--Cu with partial substitution of Pb for Bi, while a 80K phase has a 2212 composition in the same composition.
In a method of preparing an oxide superconductor, a raw material for the superconductor is filled into a metal sheath and subjected to deformation processing and heat treatment, so that the raw material, covered with the metal sheath, is brought into a superconducting state. This method is advantageously applied to preparation of a long superconducting wire, for example.
In relation to the above case, it has been recognized that the critical current density of the oxide superconductor can be improved by performing deformation processing at a high draft and repeating such deformation processing and heat treatment a plurality of times.
In order to stably use a bismuth oxide superconductor with a cooling medium of inexpensive liquid nitrogen (77.3K), it is preferable to form a 2223 phase, which is a 110K phase, as much as possible.
When a superconductor is actually applied to a superconducting wire, for example, the same must be capable of providing not only a high critical current density but also a high critical current. In order to simply attain a high critical current, for example, the thickness of the superconductor may be increased. In this case, however, the critical current density remains at a low level since the draft in the aforementioned deformation processing cannot be desirably increased and the critical current density cannot be effectively improved by the deformation processing. If the critical current density is at a low level, the same is further reduced under an external magnetic field even if the magnetic field is low. Thus, it is impossible to attain a high critical current density under the magnetic field.
Further, the critical current density of an oxide superconductor such as a bismuth oxide superconductor is generally significantly reduced when a magnetic field is applied thereto.